Method for producing recesses in a substrate

ABSTRACT

A method for producing a recess or through-opening in a substrate includes applying pulsed laser radiation to the substrate. The laser radiation is focused using an optical system at an original focal depth and, by non-linear self-focusing within the pulse duration of an individual pulse, is also focused by the optical system at a focal depth different from the original focal depth. A difference between the focal depths corresponds to or is greater than the longitudinal extent of the recess or though-opening to be produced. The laser radiation modifies the substrate along a beam axis of the laser radiation in the region of the recess or through-opening, but does not result in removal of the substrate material necessary to form the recess or the through-opening. The substrate material in the modified region is anisotropically removed to produce the recess or through-opening in the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C. §371 of International Application No. PCT/DE2014/100118, filed on Apr. 3,2014, and claims benefit to German Patent Applications No. DE 10 2013103 370.9, filed on Apr. 4, 2013, and DE 10 2013 112 033.4, filed onOct. 31, 2013. The International Application was published in German onOct. 9, 2014, as WO 2014/161534 A2 under PCT Article 21(2).

FIELD

The invention relates to a method and a device for providing a pluralityof through-holes in a substrate, which is usable as an interposer ormicrocomponent, using a laser beam. The invention further relates to asubstrate provided with through-holes which is produced in this manner.

BACKGROUND

Substrates of this type are used as interposers for electricallyconnecting the terminals of a plurality of homogeneous or heterogeneousmicrochips. The interposer generally consists of glass or silicon andcontains, for example, contact faces, rewiring connections,through-plating and active and inactive components.

As a processor core, a microchip typically has several hundred contactpoints, narrowly spaced apart from one another, distributed over arelatively small area on the lower face thereof. Because of this narrowspacing, these contact points cannot be applied directly to a circuitboard, known as the motherboard. An interposer, by means of which thecontacting base can be spread, is therefore used as a connectingelement.

In practice, a glass-fibre-reinforced epoxy resin plate, for example, isused as an interposer and is provided with a number of holes. Conductortraces extend on the upper face of the glass fibre mat, and lead intothe respective holes so as to fill them, and lead to the terminalcontacts of the processor core on the other face of the glass fibre mat.In the event of heating, however, different expansions occur in the coreprocessor and in the glass fibre mat, resulting in mechanical stressesbetween these two components.

Therefore, to reduce the stresses resulting from the different thermalexpansion coefficients, silicon interposers are also used. The siliconinterposers can be processed in the manner conventional in thesemiconductor sector. However, silicon-based interposers are veryexpensive to produce, and so efforts are increasingly being made toreplace them with more cost-effective glass material, since the thermalexpansion of glass can be matched to that of silicon.

In this context, it is found to be a challenge to process the glass intousable interposers. The prior art has not yet addressed, in particular,the economical production of the plurality of through-openings in thesubstrate for through-plating.

EP 2 503 859 A1 thus discloses a method in which a substrate is providedwith through-holes, the substrate consisting of an insulator such asglass, for example silicate glass, sapphire, plastics material orceramic and semiconductors such as silicon. The substrate is irradiatedusing a laser, for example a femtosecond laser, which is focused on afocal point at a desired position within the substrate. Thethrough-holes are produced by a method in which the regions of thesubstrate which have been modified by the laser are dipped in an etchingsolution and the modified regions are thus removed from the substrate.This etching makes use of the effect whereby the modified region isetched extremely rapidly by comparison with the unmodified regions ofthe substrate. Blind holes or through-openings can be produced in thismanner. A copper solution is suitable for filling the through-opening.To achieve a desired depth effect, in other words a through-hole betweenthe outer substrate faces, the focal point has to be displaced duringcontinuous irradiation, in other words tracked in the direction of thez-axis.

More generally, the combination of selective laser treatment with asubsequent etching process in the form of selective laser-inducedetching is also known as ISLE (in-volume selective laser-inducedetching). This method is used to produce microcomponents, tracks andshaped details in transparent materials such as glasses or sapphire. Theminiaturisation of products for microoptics, medical technology andmicrosystem technology requires the production of components havingdimensions in the micrometre range and having structural precisions ofup to 100 nm. The ISLE method is a suitable production method forstructures made of and made in transparent materials. As a result of thelaser radiation being focused in the interior of the workpiece, thematerial is structurally altered in a small volume (a few cubicmicrometres). For example, the crystalline structure of sapphire isconverted to an amorphous vitreous structure, which is etched 10,000times more rapidly than the starting material. By moving the laser focusthrough the workpiece, coherent modified areas are produced, which aresubsequently chemically etched in aqueous solution using potassiumhydroxide or hydrofluoric acid and removed.

DE 10 2010 025 966 B4 discloses a method in which in a first stepfocused laser pulses are directed onto the substrate, the radiationintensity of said pulses being high enough to result in local athermaldecomposition along a filament-like track in the glass. In a secondmethod step, the filament-like tracks are expanded into holes bysupplying high-voltage power to opposing electrodes, resulting indielectric breakdowns through the substrate along the filament-liketracks. These breakdowns expand under electrothermal heating andevaporation of hole material, until the process is halted by switchingoff the power supply upon achieving the desired hole diameter.Alternatively or in addition, the tracks may also be expanded usingreactive gases, which are directed onto the hole sites using nozzles.The through-opening sites may also be expanded using supplied etchinggas. The comparatively complex process, resulting from the fact thesubstrate initially has to be broken through by the athermaldecomposition and the diameter of the filament-like tracks has to beexpanded into holes in the following step, has proved to bedisadvantageous.

Further, U.S. Pat. No. 6,400,172 B1 discloses the introduction ofthrough-openings in semiconductor materials by laser.

SUMMARY

An aspect of the invention provides a method for producing a pluralityof recesses in a substrate using an optical system, a thickness of thesubstrate not exceeding 2 mm in a region of the recesses to be produced,the method comprising: applying pulsed laser radiation having a pulseduration (t) to the substrate material of the substrate, which istransparent at least in part to the laser wavelength, with the laserradiation being focused using the optical system having an originalfocal depth (f1) and an intensity of the laser radiation leading to amodification of the substrate along a beam axis (Z) of the laserradiation, but not to material removal which goes all the way through;and anisotropically removing material primarily in the regions whichhave previously undergone modification via the laser radiation, therebyproducing a recess in the substrate, wherein the laser radiation isfocused by the same optical system, which is unchanged per se, at afocal depth (f2) different from the original focal depth (f1), bynon-linear self-focusing within the pulse duration (t) of an individualpulse (P).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail belowbased on the exemplary figures. The invention is not limited to theexemplary embodiments. All features described and/or illustrated hereincan be used alone or combined in different combinations in embodimentsof the invention. The features and advantages of various embodiments ofthe present invention will become apparent by reading the followingdetailed description with reference to the attached drawings whichillustrate the following:

FIG. 1 is a flow chart comprising a plurality of method steps forintroducing a plurality of through-openings into a substrate byanisotropic material removal;

FIG. 2 shows a method variant in which a metal layer is applied prior tothe anisotropic material removal;

FIG. 3 shows a further method variant in which an etch resist is appliedto the substrate in a first method step;

FIG. 4 shows the intensity-dependent focal point during an individualpulse;

FIG. 5 is a graph showing the intensity distribution over time duringthe duration of an individual pulse.

DETAILED DESCRIPTION

An aspect of the invention provides an option for substantiallysimplifying a method and a device for producing through-openings, and inparticular for reducing the time taken to carry this out. Further, asubstrate produced by this method is to be provided.

The invention thus provides a method in which the laser radiation isinitially deflected through a transmissive medium, in particular aglass, the medium having a higher intensity-dependent refractive indexthan air, and is subsequently incident on the substrate, and the laserradiation is focused by non-linear self-focusing within the pulseduration of a single pulse by the unchanged optical system using a focaldepth differing from the original focal depth. The invention makes useof the fact that the intensity of a pulsed laser is not constant for anindividual pulse, but rather has an intensity which increases to amaximum and subsequently falls away over the temporal progression of theindividual pulse. Because the refractive index also increases to amaximum as a result of the increasing intensity, in a mannercorresponding to a normal distribution over the temporal progression foran individual pulse, the focal depth of the optical system, in otherwords the distance from a laser machining head or the lens, changes,independently of the geometric focal point determined by the focusingoptics. This effect is effectively amplified by the transmissive medium,in such a way that the distance between the focal points between amaximum and a minimum intensity at least corresponds to the desiredlongitudinal extent, in other words to the depth of the recess to beproduced. In a surprisingly simple manner, this results in a spatialdisplacement in the direction of the beam axis during the duration of anindividual pulse, which leads to the desired modification in the regionof the entire primary extension, in the direction of the beam axis, ofthe recesses which are subsequently to be formed. Tracking of the focalpoint, which is unavoidable in the prior art, can be omitted in thiscase. Thus, in particular, no control system for moving the laser focusthrough the substrate is required. Thus, according to the invention, notonly is the control system outlay required for this purpose omitted, butthe machining duration can also be considerably reduced, for example tothe duration of an individual pulse. The non-linear refractive index ofthe transmissive medium is a linear function of the intensity, and sothe selection of a suitable material and suitable dimensions isdependent on the intensity of the laser radiation used.

In this context, the laser beam is directed onto the substratesufficiently briefly that the substrate is merely modified along a beamaxis of the laser beam, without a recess which penetrates through thesubstrate occurring, anisotropic material removal being carried out inthe next step only in the regions of the substrate which have previouslyundergone modification by means of the laser beam, and a recess orthrough-opening thus being provided in the substrate. Although there isstill no conclusive understanding of the process according to theinvention, it is currently being assumed that the laser action duringthe modification results in a chemical modification to the substratematerial which only has slight effects on the physical properties or theexternal constitution of the substrate. The laser power input is used toexcite or trigger a reaction and a modification by conversion, theeffect of which is only made use of in the subsequent method step forthe desired material removal.

Because the anisotropic material removal is carried out by an etchingmethod, in particular by liquid etching, dry etching or vapour phaseetching, or by high-voltage or high-frequency evaporation, it ispossible to use a planar-action removal method, which only places verylow requirements on the process, rather than a sequential one for theactual material removal. Instead, over the duration of action, thematerial duration can be carried out quantitatively and qualitativelyfor all regions which are pre-treated in the described manner andcorrespondingly modified, reducing the expenditure of time for producingthe plurality of recesses or through-openings considerably overall.

The focal point at minimum intensity may be directed onto an outersurface of the substrate. However, it has already been found to beparticularly promising if the laser radiation is focused onto a remoteside of the substrate at a distance therefrom, in such a way that thefocal point of the laser radiation is set so as to be positioned on arear side, remote from the laser radiation, at a distance from thesurface of the substrate. As a result, the laser beam is initiallydirected onto a focal point positioned outside the substrate. Therefractive index which changes as a result of the increasing intensitysubsequently leads to a spatial displacement of the focal point throughthe substrate along the beam axis. This ensures that a sufficiently highintensity for producing the modification is applied to every focal pointwithin the substrate.

The duration of the beam action may, of course, comprise a plurality ofpulse durations for an unchanged relative position of the lasermachining head with respect to the substrate, for example so as furtherto optimise the modification of the substrate material. However, it isparticularly advantageous if the laser beam is deflected onto each focalpoint for the duration of a single pulse. In this way, the previous andsubsequent pulses of the laser beam are directed onto positions spacedapart in the plane of the substrate, in such a way that adjacent focalpoints are spaced apart in the plane of the substrate.

The modifications may be produced by laser machining in whichpositioning of the laser machining head and the laser machiningalternate. However, constant relative movement between the laser beam orlaser machining head and the substrate is preferably carried out whilethe laser radiation is deflected onto the substrate, in such a way thatthe laser beam is continuously guided in a “floating” movement over thesubstrate, in such a way that an interrupted change in the relativeposition results in an extremely rapid machining duration. Inparticular, the relative position of the substrate with respect to thelaser beam can be changed at a constant speed, in such a way that for aconstant pulse frequency the spacing of the modifications to be producedadheres to a predetermined grid dimension.

Particularly preferably, the laser is operated at a wavelength to whichthe substrate is transparent, ensuring penetration of the substrate. Inparticular, this ensures a substantially cylindrical modification regioncoaxial with respect to the laser beam axis, which leads to a constantdiameter of the through-opening or recess.

Further, it may also be advantageous if the laser also additionallyremoves a surface region so as to configure the action region of theanisotropic removal in a manner resulting in a conical inlet region tothe through-opening. In this manner, the subsequent through-plating issimplified. In addition, the action of an etching agent is concentratedin this region, for example.

The pulse duration can be reduced considerably by comparison with themethods known in the art. In a particularly advantageous embodiment ofthe method according to the invention, the laser can be operated at apulse duration of less than 50 ps, preferably less than 20 ps.

In another, also particularly promising embodiment of the invention, thesubstrate is provided, in particular after the modification, with aplanar metal layer covering at least some, in particular a large number,of through-openings which are subsequently to be produced. In afollowing step, the modified regions are removed in such a way that arecess sealed on one side by the metal layer is produced. The metallayer is preferably applied after the modification but before thematerial removal, in such a way that after the material removal themetal layer, applied for example as a conductor trace, seals the recessand thus simultaneously forms an optimum base for contacting to beapplied thereto. The through-plating takes place in the region of therecess by methods known per se. By applying the metal layer as aconductor trace, it is also possible to produce a desired circuitdiagram in a simple manner.

In another, also particularly promising embodiment of the method, thesubstrate is coated in a planar manner with an etch resist on at leastone surface prior to the laser treatment. As a result of the action of alaser beam, the etch resist is removed on at least one surface in adot-like action region and the modification is produced in the substratesimultaneously. In this way, the unmodified regions are protectedagainst undesired action in the subsequent etching process, and thesurface is therefore not damaged. The etch resist does not prevent themodification of the substrate positioned below. Rather, the etch resistis either permeable to the laser radiation or it is removed in a neardot-like manner by the laser radiation, that is to say evaporated, forexample. Further, the possibility is not excluded that the etch resistmay contain substances which act to promote the modification, forexample which accelerate the modification process.

Before the etch resist is applied to one of the outer surfaces of thesubstrate, the above-described metal layer may, of course, be applied soas to use it as a base for the desired through-plating after the etchresist is removed.

The etch resist may remain on the surface of the substrate after the endof the treatment. Preferably, however, the etch resist is removed fromthe surface of the substrate in a manner known per se after theanisotropic material removal.

Fundamentally, the method is not limited to particular materialcompositions of the substrate. However, it is particularly promising forthe substrate to comprise an aluminosilicate, in particular aboroaluminosilicate, as a significant material proportion.

The second object is achieved according to the invention by a devicecomprising a laser machining head for deflecting laser radiation onto asubstrate, in that the device is equipped with a transmissive medium,which in particular is provided with at least one planar face or is, forexample, configured as a planar plate, and which has a higherintensity-dependent refraction index than air, and which is arrangedbetween the laser machining head and the substrate in such a way thatthe laser radiation can be deflected through the transmissive mediumonto the substrate. As a result, according to the invention theintensity-dependent refractive index of the transmissive medium isexploited so as to produce an axial change in the focal point during theduration of each individual pulse and the accompanying fluctuation inintensity during the individual pulse, in connection with a pulsedlaser. Thus, unlike in the prior art, the focal point is not unchanged,at least during the duration of an individual pulse, but rather thefocal point is displaced along a line on the beam axis with respect tothe total duration of the individual pulse. It is easy to see whatsignificant advantages result in the present invention from the factthat the focal point is displaced without tracking of the focusingoptics of the laser machining head. In particular, this greatly reducesthe machining duration and also the control system outlay. For example,in a planar substrate the tracking of the z-axis can be omitted.

In principle, a variant is also conceivable in which the transmissivemedium is arranged on the laser machining head upstream of focusingoptics thereof in the direction of the beam path, in such a way that thelaser radiation is initially directed through the transmissive mediumand subsequently through the focusing optics onto the substrate.

The effect of the intensity-dependent light refraction can, of course,be adapted to the respective application, for example in that thetransmissive medium is adapted or replaced accordingly or in that thelaser beam passes through a plurality of transmissive media or throughthe same medium repeatedly.

The focal point may be directed onto a rear face of the substrate,remote from the laser machining head, and the transmissive medium may beformed in such a way that the intensity-dependent focal point reaches afront face, facing the laser machining head, at the intensity maximum.However, it is particularly expedient in practice if the laser radiationcan be deflected onto a focal point at a distance from a rear face ofthe substrate, remote from the laser machining head, in such a way thatthe rear face of the substrate is reached during the increasingintensity progression rather than at an intensity minimum. This ensuresa laser radiation intensity within the substrate which is alwayssufficient for the modification which is to be achieved.

In principle, any pulsed laser is suitable for the machining, a laserhaving a pulse duration of less than 50 ps, preferably less than 20 ps,having been found to be particularly expedient.

In addition, it is particularly expedient if, for focusing, the lasermachining head has focusing optics having a numerical aperture (NA)greater than 0.3, in particular greater than 0.4.

A particularly promising embodiment of the device according to theinvention is also achieved in that the focusing optics have a gradientindex lens. As a result of a lens of this type, also known as a GRINlens, the refractive index which decreases in the radial directionresults in the reduction in intensity which otherwise occurs beinggenerally compensated in the edge region of the lens.

It is further found to be advantageous if the transmissive mediumconsists of glass, in particular quartz glass, so as to provide apronounced intensity-dependent refractive index.

In this context, the transmissive medium is preferably connected to thelaser machining head and arranged so as to be movable together therewithand arranged in particular replaceably on the laser machining head.Rapid fixing, for example, is suitable for this purpose.

Preferably, the device is equipped with a continuously emitting laser inaddition to a pulsed laser, the transmissive medium being transparent tothe wavelength of the continuously emitting laser, and the continuouslyemitting laser being directed onto the substrate through the medium ordirected onto the substrate while circumventing the transmissive medium.The wavelengths of the pulsed laser and of the continuously emittinglaser may be different. Further, the laser radiation from the differentlaser sources may be directed onto the substrate from different sides.

FIG. 1 shows the individual method steps of introducing a plurality ofthrough-openings into an interposer 1, intended as a contacting elementin circuit board production, comprising a substrate 2. For this purpose,laser radiation 3 is directed onto the surface of the substrate 2. Thesubstrate 2 comprises a boroaluminosilicate as a significant materialproportion, so as to ensure thermal expansion similar to that ofsilicon. The material thickness d of the substrate 2 is between 50 μmand 500 μm. The duration of action of the laser radiation 3 is selectedto be extremely short, in such a way that merely a modification of thesubstrate 2 occurs concentrically about a beam axis of the laser beam,without resulting in significant through-destruction or a recess in thesubstrate material. In particular, the duration of action is limited tothe individual pulse. For this purpose, the laser is operated at awavelength to which the substrate 2 is transparent. A region 4 modifiedin this manner is shown in FIG. 1b . In a following method step, shownin FIG. 1c , the action of an etching liquid or etching gas (not shown)leads to anisotropic material removal in the regions 4 of the substrate2 which have previously undergone modification by the laser radiation 3.This results in a recess 5 as a through-opening in the substrate 2 alongthe cylindrical action region.

FIG. 2 shows a modification of the same method, in which, after themodification by the laser radiation 3 shown in FIG. 2b , the substrate 2is provided with a planar metal layer 6, as can be seen in FIG. 2c . Inthe next method step, shown in FIG. 2d , the anisotropic materialremoval in the modified regions 4 results in recesses 5, which aresealed on one side by the metal layer 6 and which form the basis forsubsequent contacting.

A further variant of the method is shown in FIG. 3. Prior to the lasertreatment using the laser radiation 3, the substrate 2 is coated on bothsides with an etch resist 7, shown in FIG. 3b . As a result of theaction of the laser radiation 3, removal of the etch resist 7 andmodification of the region of the substrate 2 positioned therebelow, asshown in FIG. 3c , occur simultaneously in a dot-like action region. Theunmodified regions of the surface of the substrate 2 are thus protectedagainst undesired action in the subsequent etching process, whichresults in the anisotropic material removal by a liquid etching methodand the occurrence of corresponding recesses 5 in the substrate 2, as isshown in FIG. 3d . As is shown in FIG. 3e , the etch resist 7, which issuperfluous after the end of the etching method, is removed by astripping method.

The following describes in more detail an important effect during thelaser machining of the substrate 2 with reference to FIGS. 4 and 5. Thisis the intensity-dependent focal point during an individual pulse P. Theinvention is based on the finding that the intensity I of an individualpulse P of the laser radiation 3 is not constant, but rather has anintensity which increases from a minimum I_(a) through an average I_(b)to a maximum I_(c) and subsequently decreases over the temporalprogression of the individual pulse as shown in FIG. 5, for example, inaccordance with a normal distribution. Simultaneously, as a result ofthe variable intensity I, the refractive index, in particular also of atransmissive medium 8, changes in relation to an individual pulse P overthe temporal progression t. As a result, the intensity-dependent focalpoints 9 a, 9 b, 9 c of the laser radiation 3, which are shown in FIG.4a to 4c , also change independently of the geometric focal pointdetermined by focusing optics of a laser machining head 10. This effectis amplified by the transmissive medium 8, for example made of glass,which is arranged between the laser machining head 10 and the substrate2 and which has a greater intensity-dependent refractive index than air,in such a way that the distance between the focal points 9 a, 9 cbetween a maximum intensity I_(c) and a minimum intensity I_(a) at leastcorresponds to the desired longitudinal extension, in other words to thedepth of the recess to be produced or, if, as shown, a through-hole isto be produced, to the material thickness d of the substrate 2. Theintensity-dependent focal point 9 a, 9 b, 9 c thus migrates along thebeam axis Z from a position, which is shown in FIG. 4a and is at adistance from a rear face 11 of the substrate 2, in the direction of thelaser machining head 10, and thus reaches all positions along the beamaxis Z between the rear face 11 and a front face 12 facing the lasermachining head 10 in a continuous movement, in such a way that thedesired modification occurs in the region of the entire primaryextension of the recesses which are subsequently to be produced.

Additionally, FIG. 4a shows, merely schematically, an additional lasermachining head 13, which, to supplement a continuously emitting lasersource connected to the laser machining head 10, directs the laserradiation 3 of a pulsed laser onto the substrate 2 selectively throughor circumventing the transmissive medium 8. As a result, the intensityI, shown in FIG. 5, of an individual pulse P of the laser radiation 3 isaccordingly amplified by the intensity of the continuously emittinglaser source.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Itwill be understood that changes and modifications may be made by thoseof ordinary skill within the scope of the following claims. Inparticular, the present invention covers further embodiments with anycombination of features from different embodiments described above andbelow. Additionally, statements made herein characterizing the inventionrefer to an embodiment of the invention and not necessarily allembodiments.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B, and C” should be interpreted as one or more of agroup of elements consisting of A, B, and C, and should not beinterpreted as requiring at least one of each of the listed elements A,B, and C, regardless of whether A, B, and C are related as categories orotherwise. Moreover, the recitation of “A, B, and/or C” or “at least oneof A, B, or C” should be interpreted as including any singular entityfrom the listed elements, e.g., A, any subset from the listed elements,e.g., A and B, or the entire list of elements A, B, and C.

The invention claimed is:
 1. A method for producing a recess or athrough-opening in a substrate using an optical system, a thickness ofthe substrate not exceeding 2 mm in a region of the recess or thethrough-opening to be produced, the method comprising: applying pulsedlaser radiation having a pulse duration to the substrate material of thesubstrate, which is transparent at least in part to the laserwavelength, wherein the laser radiation is focused using the opticalsystem at an original focal depth and, by non-linear self-focusingwithin the pulse duration of an individual pulse, is also focused by theoptical system at a focal depth different from the original focal depth,a length between the focal depths being greater than and extendingacross the thickness of the substrate in the region of the recess or thethough-opening to be produced, whereby an intensity of the laserradiation modifies the substrate along a beam axis of the laserradiation in the region of the recess or the through-opening, but doesnot result in removal of the substrate material necessary to form therecess or the through-opening, wherein the laser radiation is deflectedthrough a transmissive medium having a higher intensity-dependentrefractive index than air, the transmissive medium being arrangedbetween the substrate and a laser machining head emitting the laserradiation such that the distance between the focal depths between amaximum intensity and a minimum intensity of the individual laser pulseis greater than the predetermined longitudinal extent of the recess orthe though-opening to be produced; and anisotropically removing thesubstrate material in the modified region of the recess or thethrough-opening so as to produce the recess or the through-openingextending into the substrate in a direction of the thickness.
 2. Themethod of claim 1, wherein the focal depth is less than the originalfocal depth.
 3. The method of claim 1, wherein the substrate comprisesglass, sapphire, and/or silicon, as a significant material proportion.4. The method of claim 1, wherein the anisotropically removing comprisesetching in hydrofluoric acid.
 5. The method of claim 1, furthercomprising: coating the substrate with an etch resist on at least oneside prior to the laser treatment.
 6. The method of claim 5, furthercomprising: removing the etch resist from the surface of the substrateafter the anisotropically removing.
 7. The method of claim 1, furthercomprising: coating the substrate is carried out in a planar manner withan etch resist on at least one surface prior to the laser treatment; andremoving, as a result of the action of the laser radiation, the etchresist in a dot-like action region, thereby producing the modificationin the substrate on the at least one surface simultaneously.
 8. Themethod of claim 1, further comprising: coating the substrate with ametal on at least one side.
 9. The method of claim 8, wherein thecoating comprises providing the substrate, after the laser treatment,with a planar metal layer covering at least the recess or thethrough-opening to be produced such that the subsequent anisotropicallyremoving produces the recess or the through opening sealed on one sideby the planar metal layer.
 10. The method of claim 1, wherein a numeralaperture (NA) of the optical system is greater than 0.3.
 11. The methodof claim 1, further comprising: deflecting the laser radiation through afocusing subsystem of the optical system and through a material of theoptical system having a higher intensity-dependent refractive index thanair, wherein the material transmits the laser wavelength of the laserradiation.
 12. The method of claim 1, wherein the transmissive mediumcomprises sapphire.
 13. The method of claim 1, wherein the originalfocal depth is disposed past a rear face of the substrate remote in thedirection of the laser radiation along the beam axis.
 14. The method ofclaim 1, wherein the focal depths are separated by at least 30 μm in adirection of the beam axis of the laser radiation during the pulseduration of the individual pulse.
 15. The method of claim 1, furthercomprising, while the laser radiation is acting on the substrate: movingthe substrate relative to the laser radiation and/or a laser machininghead emitting the laser radiation.
 16. The method of claim 1, wherein aplurality of recesses or through-holes are produced by the opticalsystem being moved relative to the substrate at a uniform relativespeed.
 17. The method of claim 1, wherein the laser radiationadditionally removes a surface region.
 18. The method of claim 1,wherein the pulse duration of the laser radiation is less than 50 ps.19. The method of claim 1, wherein the original focal depth ispositioned outside of the substrate, and wherein, during focal depthmovement by the non-linear self-focusing, the intensity of theindividual laser pulse increases from the minimum intensity at theoriginal focal depth to the maximum intensity at the focal depthdifferent from the original focal depth.
 20. The method of claim 1,wherein the anisotropically removing is performed so as to form thethrough-opening extending entirely across the thickness of thesubstrate.